diff --git a/compute_hyperv/nova/volumeops.py b/compute_hyperv/nova/volumeops.py
index 93b04145..8f9ff577 100644
--- a/compute_hyperv/nova/volumeops.py
+++ b/compute_hyperv/nova/volumeops.py
@@ -410,10 +410,13 @@ class BaseVolumeDriver(object):
         # on this tag.
         if not self._is_block_dev:
             disk_path = self.get_disk_resource_path(connection_info)
+            # In this case, we're not tagging the disks, so we want os-win
+            # to use the disk path to identify the attachment.
+            serial = None
         else:
             disk_path = None
+            serial = connection_info['serial']
 
-        serial = connection_info['serial']
         LOG.debug("Detaching disk from instance: %(instance_name)s. "
                   "Disk path: %(disk_path)s. Disk serial tag: %(serial)s.",
                   dict(disk_path=disk_path,
diff --git a/compute_hyperv/tests/unit/test_volumeops.py b/compute_hyperv/tests/unit/test_volumeops.py
index eea24857..4cb56a7f 100644
--- a/compute_hyperv/tests/unit/test_volumeops.py
+++ b/compute_hyperv/tests/unit/test_volumeops.py
@@ -624,9 +624,11 @@ class BaseVolumeDriverTestCase(test_base.HyperVBaseTestCase):
                                             mock.sentinel.instance_name)
 
         if is_block_dev:
+            exp_serial = connection_info['serial']
             exp_disk_res_path = None
             self.assertFalse(mock_get_disk_resource_path.called)
         else:
+            exp_serial = None
             exp_disk_res_path = mock_get_disk_resource_path.return_value
             mock_get_disk_resource_path.assert_called_once_with(
                 connection_info)
@@ -635,7 +637,7 @@ class BaseVolumeDriverTestCase(test_base.HyperVBaseTestCase):
             mock.sentinel.instance_name,
             exp_disk_res_path,
             is_physical=is_block_dev,
-            serial=connection_info['serial'])
+            serial=exp_serial)
 
     def test_get_disk_ctrl_and_slot_ide(self):
         ctrl, slot = self._base_vol_driver._get_disk_ctrl_and_slot(